Combustion exhaust gas (to be referred to as “exhaust gas” hereinafter) discharged from boilers in thermoelectric power stations and waste incinerators generally contains sulfur dioxide (SO2). Sometimes, it also contains mercury to a high-concentration level depending on the type of fossil fuel (coal in particular) and the types of wastes that are burnt there. Since those substances are harmful substances that adversely affect the health of inhabitants if discharged into the environment, they need to be removed before exhaust gas is emitted into the atmosphere from those facilities. Of the above listed substances, sulfur dioxide has been and being required to be removed by exhaust gas release control regulations. Recently, regulations for compulsorily removing mercury from exhaust gas have been enforced in some countries.
Methods of removing sulfur dioxide (SO2) from exhaust gas include wet methods of causing absorption liquid to absorb and remove sulfur dioxide and dry methods of causing an adsorbent to adsorb and remove sulfur dioxide. While a number of methods are known for both wet methods and dry methods, wet methods are generally employed to process a large quantity of exhaust gas containing sulfur dioxide to a high-concentration level from the viewpoint of treating cost.
Mercury in exhaust gas contains both Hg2+, or divalent mercury compounds, produced as mercury is oxidized by combustion in incinerators and catalysts of flue gas denitrators (SCR) and Hg(0), or elemental (zero-valent) metal mercury, of which Hg2+ is removed almost entirely in the wet scrubber of a flue gas desulfurization unit but Hg(0) is poorly removed to represent a low removal efficiency because it is hardly dissolved in absorption liquid. In other words, most of Hg(0) in exhaust gas is currently not removed and emitted into the atmosphere.
A method of removing Hg(0) by adding active carbon powder into exhaust gas and causing the latter to be adsorbed by the former is known (Japanese Patent Application Laid-Open No. 09-308817). However, a large facility including equipment for ejecting active carbon powder that is solid into exhaust gas, a large electric dust collector (or electric precipitator, EP) for capturing at a downstream side active carbon dispersed in exhaust gas along with fly ash and a device for processing the captured active carbon that is mixed with fly ash is required to carry out this method to make the entire exhaust gas treating facility complex and costly. While the mercury removing ability may expectedly be enhanced when halogen such as iodine or bromine is made to adhere to active carbon, sulfuric acid produced from sulfur dioxide is apt to be accumulated on active carbon when halogen is made to adhere to active carbon so that the reality is that the mercury adsorbing ability is not raised to such an expected level.
A method of raising the extent of oxidizing Hg(0) in exhaust gas to Hg2+, by adding one or more halogen compounds such as hydrogen chloride (HCl) and/or calcium bromide (CaBr2) to the coal that is burnt as fuel or by utilizing the oxidation catalyst in a denitrator has also been proposed (Japanese Patent Application Laid-Open No. 2004-66229). However, this method has a drawback of shortening the service life of the catalyst and it is difficult to achieve a high oxidation rate because of the diffusion limited access of Hg(0). Thus, it is difficult to oxidize Hg(0) to Hg2+ stably at high efficiency for a long period of time, accommodating fluctuations in the coal types and the combustion conditions.
Meanwhile, a method of adding a mercury fixing agent such as a chelating agent or a potassium iodide (KI) solution or an oxidizing agent such as hypochlorous acid (HClO) or hydrogen peroxide (H2O2) to the absorption liquid in a wet type flue gas desulfurization unit is known (Japanese Patent Application Laid-Open No. 10-216476). However, this method is accompanied by a problem that the mercury solidifying agent or the oxidizing agent is consumed and decomposed in a reaction with some other metal and in oxidation of SO2 in exhaust gas and also volatilized and discharged from the flue to consequently increase the consumption of such additives. Additionally, chelating agents give rise to a problem of being decomposed to produce hydrogen sulfide (H2S) and give off a foul smell.
Additionally, as for the method of adding various additives to absorption liquid, it is known that the state of the absorption liquid changes as the load of power generation and the composition of exhaust gas fluctuate so that consequently some of the Hg(0) that is absorbed is emitted or Hg2+ is reduced to become Hg(0), which is then emitted again. Techniques of not releasing Hg(0) again are being developed in view of this problem (Japanese Patent Application Laid-Open No. 2004-313833). Furthermore, with techniques of using an oxidizing agent such as hypochlorous acid, hydrogen peroxide, chromic acid (H2CrO4) or chlorine (Cl2), the oxidizing agent inevitably reacts with SO2 in exhaust gas to be consequently consumed to a large extent. Therefore, a technique of spraying such an oxidizing agent at the gas downstream side of the flue gas desulfurizer has been proposed (Japanese Patent Application Laid-Open No. 2001-162135).